1. Field of the Invention
The present invention relates to an image forming apparatus that forms a toner image by using a developer acquired by adding an external additive to toner.
2. Description of the Related Art
Conventionally, as toner used for an image forming apparatus, in order to provide fluidity, toner to which a fluidizing agent such as silica, titania, or alumina is added is used. When the fluidizing agent (hereinafter, referred to as an “external additive”) such as silica is added, the fluidity of the developer can be configured to be appropriate, and the generation of toner aggregates, deterioration of the image quality, and the like accompanied with a decrease in the fluidity can be suppressed.
However, as the period of the use increases, the external additive attached to the toner surface receives a share accompanied with stirring or conveyance in a development device, and there is concern that the external additive is gradually separated from the toner or is buried in the toner surface. It is difficult to completely prevent such degradation, and it is predicted that the degradation will becomes more marked in the ongoing trend of increasing the speed of the image forming apparatus. When the degradation occurs, the fluidity providing function of the external additive disappears, and there is concern of aggregates being generated, the image quality being degraded, and the like. Such a problem may easily occur particularly for a low image ratio at which the amount of consumption of the toner is small. The reason for this is as follows.
Generally, in an image forming apparatus, a method is widely used in which toner of a predetermined amount is input to a development device in advance, and, when the toner is consumed for forming an image, toner of an amount corresponding to a decrease is supplied. In such a case, even when toner disposed inside the development device is degraded, the toner disposed inside the development device is gradually replaced with new toner in the process of consuming the toner and newly supplying toner. Accordingly, the degradation of the toner can be suppressed. However, in a case where the amount of consumption of the toner is small, new toner is not supplied that much, and the toner is not replaced with new toner. In such a case, since the toner stays in the development device for a long time, the degradation of the toner may easily progress.
In order to deal with such a problem, an image forming apparatus having a mode for forcibly consuming toner at a time other than the time of forming an image in a case where the amount of consumption of the toner is small has been proposed and used (for example, see Japanese Patent Laid-Open No. 2006-337699). In such a configuration, in a case where the amount of consumption of the toner is determined to be small, the toner is additionally consumed in a forced manner, and accordingly, also degraded toner is gradually replaced with new toner, whereby a reduction in the degradation is suppressed.
However, in such a configuration, a time is required for forcibly consuming the toner at a time other than the time of forming an image. For this reason, there is a problem in that a down time increases for a user. In addition, since toner is unnecessary consumed, there is a problem in terms of cost. Accordingly, a configuration in which such forced consumption of toner does not need to be performed is desirable.
As a countermeasure that can be performed without forcibly consuming the toner as described above, a configuration in which a large amount of an external additive is added to the toner in advance may be considered. In a case where a large amount of the external additive is added in advance, even when the external additive is gradually separated as the used time elapses, there is a delay until the external additive completely disappears. Accordingly, the fluidity providing function of the external additive can be maintained for a longer time. As a result, the occurrence of a problem such as degradation of the image quality accompanied with a reduction in the fluidity can be suppressed.
However, also in such a configuration, there are the following problems. In order to acquire a satisfactory effect from the configuration described above, while it is necessary to increase the amount of the external additive, on the other hand, in a case where the amount of the external additive of toner is increased, there is concern that the fluidity becomes too high particularly in the initial period. While low fluidity of toner causes a problem, in a case where the fluidity of toner is excessively high, other problems occur such as a reduction in the conveyance capability of a developer on a developing sleeve. For this reason, it is necessary to suppress the fluidity of the initial period to be in an appropriate range to some degree. Accordingly, practically, there is an upper limit of the amount of the external additive to be added to toner in advance.
The above-described problem is originated from that toner (hereinafter, referred to as “initial toner”) put into a developing container in the initial period and toner (hereinafter, referred to as “replenishment toner”) supplying a consumed amount are toner of the same kind. Thus, a proposal for resolving the above-described problem by changing the external added amount between the initial toner and the replenishment toner has been made (see Japanese Patent Laid-Open No. S61-228460 and H7-319200).
In Japanese Patent Laid-Open No. S61-228460 and H7-319200, configurations have been proposed in which the external added amount of the replenishment toner is increased with respect to the external added amount of the initial toner. Since the external added amount of the replenishment toner is increased, as described above, even when the external additive is gradually separated according to the elapse of the used time, it is possible to maintain the fluidity providing function of the external additive for a longer time. For this reason, degradation of the image quality that is accompanied with a decrease in the fluidity can be suppressed. On the other hand, since the amount of the external additive in the initial toner is suppressed to be small, like the case described above, there is no concern that the fluidity becomes too high in the initial period. Accordingly, a decrease in the fluidity after an elapse of the used time can be suppressed while the level of the fluidity of the initial period is suppressed within an appropriate range.
However, by increasing the amount of the external additive of the replenishment toner as in the above-described configuration, in the case of a high image ratio at which the amount of consumption of the toner is large, there is concern that a new problem as represented below is caused.
In the process of consuming the toner and supplying new toner, some of the external additive added to the toner is peeled off to fall in the development device. When a large amount of the external additive peeled off to fall as above is accumulated in the development device, various properties of the toner are influenced thereby. The external additive has influences on the charged level and the environment dependency of the toner and the like in addition to the fluidity provision. For this reason, when a large amount of the external additive is accumulated, there is concern that there is a problem in the stability through the elapse of the used time such as the charged level and the environment dependency of the toner.
In addition, in a case where a two-component developer in which a carrier is mixed together with toner is used, when a large amount of the external additive is accumulated, the external additive transits to the carrier. Then, there is a possibility that the charged level of the toner according to the carrier is influenced. Such a problem according to the accumulation of the external additive becomes remarkable at a high image ratio at which the amount of consumption of the toner is large. The reason for this is that the amount of the external additive peeled off to fall increases as a larger amount of toner is supplied by the development device.
As above, in a case where the amount of the external additive in the supply toner is increased so as to solve the problem of a decrease in the fluidity at a low image ratio, the problem of the accumulation of the external additive at a high image ratio becomes remarkable.
As a countermeasure for the above-described problem, it may be considered to suppress the accumulation of the external additive at a high image ratio by decreasing the amount of the external additive in the same way by improving the fluidity providing function of the external additive. However, in a case where the added amount of the external additive in the initial toner is small, there is concern as described below.
As described above, in a case where a large amount of the external additive is added, even when the external additive is gradually separated as the used time elapses, there is a delay in time until the external additive completely disappears. For this reason, the fluidity providing function of the external additive can be maintained for a long time. Conversely, in a case where the amount of the added external additive is small, particularly, at a low image ratio, the external additive is gradually separated as the used time elapses, and there is no delay in the time until the external additive completely disappears. Thus, there is concern that the image is degraded according to a decrease in the fluidity at a low image ratio.
In a case where the image ratio used by a user is known in advance, the problems described above can be responded by adjusting the amount of the external additive. However, since the image ratio for a user is not known in advance, and, practically, the use method differs much depending on the user, it is not possible to practically respond to an individual case.